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GUIDANCE FOR INSPECTION OF ATMOSPHERIC, REFRIGERATED AMMONIA STORAGE TANKS
2008 european fertilizer manufacturers association
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GUIDANCE FOR INSPECTION OF ATMOSPHERIC, REFRIGERATED AMMONIA STORAGE TANKS SECOND EDITION (First Edition 2002)
Copyright 2008 EFMA EFMA European Fertilizer Manufacturers Association Avenue E. van Nieuwenhuyse 6 B-1160 Brussels Belgium
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CONTENTS 1. SCOPE
4
2. INTRODUCTION
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3. DESCRIPTION OF SPECIFIC AREAS OF CONCERN 3.1 Ammonia Storage Facilities 3.2 Types of Ammonia Storage Tanks 3.3 Ancillary Equipment 3.4 Design and Materials of Construction 3.4.1 Materials of Construction 3.4.2 Pressure Relief Devices 3.4.3 Construction Documentation 3.5 Factors Affecting the Integrity of Ammonia Storage Tanks 3.5.1 Original Weld Defects 3.5.2 General Corrosion 3.5.3 Stress Corrosion Cracking 3.5.4 Fatigue 3.6 Indications from Accidents
6 6 11 12 12 12 12 12 13 13 13 17 17
4. INSPECTION STRATEGY
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5. INSPECTION 5.1 Competence and Independence 5.2 Assessment for Inspection Frequency 5.3 Procedure for Structural Integrity Calculations and Definition of Inspection Programme 5.4 Integrity Inspection from Inside 5.5 Non-Intrusive Integrity Inspection (from outside) 5.5.1 General Comments 5.5.2 Non-Intrusive In-Service Inspection Methods 5.5.3 Number, Size and Location of Areas to be Inspected 5.6 Other Inspection Issues 5.7 Reporting
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21 22 24 25 26 26 28 30 33 34
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6. EVALUATION, REPAIRS AND CORRECTIVE ACTIONS 6.1 Evaluation 6.2 Repairs 6.3 Corrective Actions
35 35 35
7. COMMISSIONING, DECOMMISSIONING AND RECOMMISSIONING
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8. GLOSSARY & EXPLANATION OF TERMS
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9. REFERENCES
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APPENDICES APPENDIX 1
Welding consumables for ammonia tank construction
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APPENDIX 2
Risk based inspection evaluation
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APPENDIX 3
Crack configurations that should be evaluated by structural integrity calculations
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Analysis of oxygen in liquid ammonia
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APPENDIX 4
Disclaimer The information and guidance in this Booklet is given in good faith. The European Fertilizer Manufacturers’ Association (EFMA), its consultants, its member companies and their staff accept no liability for any incident, loss, damage or any other consequences arising from the use, misuse, practical application of or reliance on the information given in this document. Users of this Booklet are advised to consult their latest national regulations before carrying out their own inspections, as changes in the regulations may have been made since its publication. 3
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1. SCOPE This document, produced by EFMA, provides guidance for the periodic in-service inspection of fully refrigerated anhydrous liquid ammonia storage tanks, which operate at or near atmospheric pressure and -33°C and are located in Europe. The Guidance focuses on major periodic inspection, covering its periodic frequency, method of inspection and regular monitoring between major inspections. It does not cover fabrication inspection. In considering the inspection frequency it describes as an option a risk based inspection (RBI) approach requiring the evaluation of the probability and consequences of failure for each individual tank. The underlying intention is to maximise the operational safety and reliability of these tanks.
2. INTRODUCTION The practice of the inspection of storage tanks, which contain anhydrous liquid ammonia at atmospheric pressure, is not uniform in various countries in Europe. One of the reasons for this is that commonly used regulations relating to pressurised systems do not apply to these storage tanks; because they essentially operate at atmospheric pressure. Whereas in some countries e.g. Austria and Belgium there are regulations specifying the frequency of inspection for these tanks, in some other countries industry codes have been prepared for this purpose e.g. United Kingdom [Ref. 1]. On the other hand, in several other countries e.g. Germany, Greece, Italy and Portugal, there are no specific regulations or codes concerning inspection requirements for these tanks. Some companies have their own internal standards or they supplement the national regulations or industry codes with their own internal standards or codes of practice. Ammonia storage tank systems have to comply with a number of more general safety regulations in most countries. Of particular importance in this regard is the need to comply with specific regulations arising from the SEVESO Directive [Ref. 2], which specifies several safety related requirements relating to process operations including maintenance. In revising this Guidance, EFMA carried out two types of surveys of tanks operated by its members. The first type, which covered 22 tanks, dealt with the design and construction aspects of the tanks and was the basis for Chapter 3. It showed that virtually all tanks have some form of secondary containment provision to retain liquid in the event of a failure. Of these, more than 80% are of full height concrete or steel wall construction. Most of the tanks have a single roof, whereas some tanks have two independent roofs. In Europe, there are more than 50 refrigerated ammonia storage tanks in operation. The second more detailed survey, based on 48 tanks, covered factors which affect failure probability and failure consequences. The results of this survey provided the basis for the Risk Based Inspection (RBI) matrix explained in Chapters 4 and 5. The main purpose of this document is to provide guidance and recommendations for the periodic inspection of fully refrigerated anhydrous liquid ammonia storage tanks. 4
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The Guidance is based on experience gained from inspection of ammonia tanks and the knowledge of potential failure mechanisms, which can affect the integrity of the tanks, in particular, stress corrosion cracking (SCC) induced by ammonia under certain conditions. The Guidance covers the three main stages in the overall process of inspection management viz, determination of periodic frequency by legislation, industry code or a risk based inspection (RBI) approach or other options, methods of major inspections (intrusive and non-intrusive) and monitoring between inspections. Figure 1 summarises this overall approach.
Is the storage tank for fully refrigerated liquid ammonia?
No
This Guidance not applicable
Yes Is the tank vertical cylindrical type?
No
Yes Is there a requirement from the national legislation or regulatory body concerning inspection frequency?
Yes
RBI approach acceptable to authorities?
No Is there in-house company guidance or national/international industry code (other than EFMA) which the company policy requires following No Does the company follow EFMA Guidance? No Establish company policy for inspection following consultation with regulatory bodies.
Discuss with the authorities RBI approach
No
Follow existing regime
Yes Yes Develop Risk Based Inspection RBI) Programme See EFMA Guidance sections 4 & 5
Determine inspection frequency for the tank in question. See section 5. Determine suitable inspection method taking account of tank construction aspects.
Figure 1 Overall Approach for Inspection 5
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The Guidance describes the RBI approach as a way to optimise the inspection programme between the need for knowledge about the condition of the tank and the negative effects of opening the tank for inspection which could increase the potential for SCC. Risk based inspection involves the planning of an inspection on the basis of the information obtained from a risk analysis of the equipment. The purpose of the risk analysis is to identify the potential degradation mechanisms and threats to the integrity of the equipment and to assess the consequences and risks of failure. The inspection plan can then target the high risk equipment and be designed to detect potential degradation before fitness for service could be threatened. The process of risk based inspection should form part of an integrated strategy for managing the integrity of the systems and equipment. Application of these recommendations requires an appropriate level of competence and experience of ammonia storage tank design and operations.
3. DESCRIPTION OF SPECIFIC AREAS OF CONCERN 3.1 Ammonia Storage Facilities Liquid ammonia is stored either at ambient temperature under high pressure or at -33°C under atmospheric pressure. (The description liquefied is also sometimes used for liquid, see Glossary for explanation). In some cases, it is also stored at intermediate temperatures and pressures (semi-refrigerated). For pressure vessels, the inspection requirements in most countries are governed by the respective pressure vessel codes and regulations. The recommendations provided in this Guidance are, therefore, limited to atmospheric pressure storage tanks, which operate at -33°C.
3.2 Types of Ammonia Storage Tanks Illustrations of different types of storage tanks are shown below. The main types of atmospheric tanks operating at -33°C in Europe are: a) Steel tank with full height concrete bund wall close to it with capacity to contain the full contents of the tank and the space between the tank and the bund having an impervious floor and roof covering (see Figure 2). b) Steel tank housed within another steel tank to contain the full contents of the tank, with a single roof (cup in tank) or independent roofs (see Figure 3). c) Steel tank with a partial height concrete bund wall with impervious floor within the contained area and no roof over the space (see Figure 4). d) Steel tank with an embankment of earth to contain the full contents of the tank and no roof over the space between the tank and the embankment (see Figure 5). e) Single steel wall tank with no secondary containment (see Figure 6). 6
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Figure 2 Tank with full height concrete bund
Picture 1 Steel tank surrounded by a concrete wall 7
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Figure 3 Tank with steel outer and inner walls with separate roofs
Picture 2 Tank with steel outer and inner walls with separate roofs 8
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Figure 4 Tank with remote concrete bund
Figure 5 Tank with bund of earth dyke
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Picture 3 Single steel wall tank with earth dyke
Figure 6 Tank with no secondary containment 10
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As can be seen from the figures above, there are two main types of foundation: • tank resting on concrete plinths such that the ground below is not exposed to freezing conditions due to ammonia; therefore, the heating of the ground below the tank base is not necessary • the tank sits on a suitable foundation directly on the ground. This arrangement requires heating of the foundation to prevent it freezing. Bund is a wall of brick, stone, concrete or other suitable material or an embankment of earth, which provides a barrier to retain liquid. Since the bund is the main part of a spill containment system, the whole system (or bunded area) is generally referred to within industry as the “bund”. Its capacity and strength should be designed so as to be capable of containing liquid ammonia that may be released from the (full) tank in an accidental situation. It should be able to contain spillages and leaks of ammonia stored or processed above-ground and to facilitate clean-up operations. A bund generally consists of: • an impervious bund wall or embankment surrounding the facility or tanks • a floor (preferably impervious) within the bunded area • any joints in the floor or the wall or between the floor and the wall • any associated facilities designed to remove liquids safely from the bunded area without polluting the environment.
3.3 Ancillary Equipment It is expected that the tank operation is in accordance with best available operating procedures based on HAZOP or similar process risk evaluation tools. The design of individual storage tanks and their associated ancillary equipment can vary between installations. Typical items that require systematic attention during life time of tanks include: • Relief valves. • Nozzles. • Drainage systems. • Insulation: at the roof, wall and in the bottom. • Heating system for foundations (where installed). The procedure described in this document is not considered to be valid if these items are not effectively operated and maintained. Where appropriate, they should be included as part of a systematic schedule for maintaining the tank and its associated ancillary equipment.
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3.4 Design and Materials of Construction Tanks for the storage of anhydrous ammonia at or near atmospheric pressure and -33°C will normally be designed according to a suitable design code such as API 620 R: Design and Construction of Large, Welded, Low-Pressure Storage Tanks [Ref. 3], EN 14620 [Ref. 4]; or similar codes. 3.4.1 Materials of construction Materials for atmospheric ammonia tanks are selected to satisfy the requirements specified in the design codes. The standard type of material is low temperature certified carbon manganese steel, impact tested at or near -40°C. The susceptibility to stress corrosion cracking increases with increasing yield strength of the steel. Materials with minimum yield strength between 290 and 360 MPa are often used. For new tanks, the use of material with minimum yield strength in the lower part of the above-mentioned range is recommended. Various types of welding materials are used in construction, but often with a considerably higher strength level than the base material. Compatibility of yield strength level between weld and base material is an important parameter for resistance against ammonia stress corrosion cracking. Some typical data for welding consumables are shown in Appendix 2. 3.4.2 Pressure relief devices There are a number of industry codes which specify the design of pressure relieve devices e.g. API 620/API 2000, EN 14620. These requirements should be applied to the construction of new tanks. Since the inspection frequency for pressure relief devices is higher in most cases than that of the tank, due care shall be given to the inspection and testing requirements of these devices in order to prevent interference with the inspection regime of the tank itself. 3.4.3 Construction documentation It is important that detailed records are kept of the quality inspection activities during tank construction and fabrication in order to enable an accurate RBI evaluation to be carried out, in particular material toughness properties.
3.5 Factors Affecting the Integrity of Ammonia Storage Tanks As with all other constructions, ammonia tanks can be affected by their internal and/or external environment. Ammonia is not generally corrosive to the materials selected for tank construction. The contaminants normally found are oil and water, but the quantities are normally small. With regard to water, this inhibits SCC and therefore has a positive effect to service life. Oil has no negative effect on service life.
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3.5.1 Original weld defects Ammonia storage tanks are constructed according to appropriate design standards, such as API 620 R, EN 14620 or equivalent. These standards have requirements for the inspection of welds by radiographic (RT) and magnetic testing (MT) to ensure the quality of the welds is of the required standard,. The quality and integrity of the welds prior to first commissioning are vital for the future life of a tank, particularly in the initiation and propagation of SCC under ammonia duty. Residual stresses and local hardness peaks should be minimised by sound welding procedures and the appropriate heat treatment. 3.5.2 Corrosion External corrosion of the tank due to atmospheric conditions is prevented by appropriate paint and/or by the application of insulation containing a vapour membrane that reduces the ingress of atmospheric moisture. It is worth noting that at the storage temperature of -33°C the corrosion rate is negligible. The roof may be attacked externally by general corrosion, particularly where the insulation is inside the tank and consequently the roof tends to be close to atmospheric temperature. The roof should be regularly inspected and, where possible, repaired without interruption of service. It is important that the condition and integrity of the insulation and vapour membrane on all areas of the tank are considered as part of the overall inspection assessment. The ingress of oxygen during the emptying of the tank, or caused by leakage in the safety valves can theoretically cause some corrosion in the upper part of the wall. However, in practice, oxygen is effectively removed because of the continuous cooling by compression. No detectable deterioration has therefore been found internally due to general corrosion. 3.5.3 Stress corrosion cracking Stress corrosion cracking is a phenomenon which can occur in metals exposed to a combination of stress and corrosive environment. The corrosive environment will, under certain circumstances, destabilise the protective oxide layer, without causing general corrosion. This destabilisation is sufficient to prevent the reformation of oxide after a crack, caused by stress. Liquid ammonia in the presence of oxygen can cause SCC in carbon steels. The probability of SCC increases with increasing yield strength of the plate material, increasing strength of the weld metal and local hardness in the welds. The stress levels required to initiate such cracking are high and are not experienced during normal operations. However the residual welding stress levels in high and medium strength materials or welds with over matched strength, together with the applied stresses, can be enough to initiate SCC if oxygen is present in sufficient quantities.
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Picture 4 Cross section of crack caused by SCC Since the late 1980s stress corrosion cracking has been detected in some storage tanks operating at -33°C. Based on experiences from findings and extensive international research work, it appears that the commissioning and to an even greater extent recommissioning are critical phases in the formation of cracks. This is due primarily to the potential for increased oxygen levels inside the tank and temperature variations causing increased stress levels. Much research work has been carried out to understand the SCC mechanism and the relevant factors [Refs. 5-16]. The main conclusions concerning SCC in ammonia tanks from this work combined with practical experience are: 1) SCC is difficult to initiate at -33°C. 2) SCC initiation requires applied and/or residual stress levels greater than the yield stress. 3) SCC initiation requires the presence of oxygen. 4) The presence of water inhibits the formation and growth of SCC. 14
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Picture 5 Three stress corrosion cracks in the welding zone 5)
Where SCC is found in low temperature tanks, the defects are in general very small (less than 2 mm deep). However, a few exceptions with larger defects have been reported. 6) Commissioning and in particular recommissioning is a critical period for the formation and growth of SCC. 7) Knowledge and experience of SCC has led to the improved operation of ammonia storage tanks. Due to this, recent experience indicates that the problem occurs less frequently, even in tanks where extensive cracking has been detected earlier. The phenomenon of SCC is rare in low temperature tanks due to the need for the presence of oxygen to catalyse the process and the low temperature that slows the process. The dependence of SCC on water content and oxygen concentration in ammonia is shown in Figure 7. Information on the method of analysing oxygen in ammonia is given in Appendix 4.
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Experiments at lower temperatures (-33°C) show that SCC can occur in about the same range of oxygen and water content compared to ambient temperatures, although it is much more difficult to initiate stress corrosion cracks at -33°C than at ambient temperature.
Figure 7 Relationship between water and oxygen content of ammonia and the risk of SCC Actions to improve service life by shot-peening or cathodic protection are considered to be non proven technology and hence have not been included as beneficial for protection against SCC in ammonia tanks. Quantification of the probability that a critical crack may develop is based on documented experience with ammonia tanks. A few hundred tanks are estimated to be in operation worldwide, representing about several thousand tank years. Although properly documented inspection results have only been published for relatively few tanks, it is reported that 5 fully refrigerated ammonia storage tanks in Europe had developed ammonia stress corrosion cracks [Refs. 17 and 18]. It should be noted however that critical defect sizes can vary between tanks due to variations in the strength and fracture toughness properties of the actual weld and plate materials, applied stress and residual stress levels. SCC in fully refrigerated ammonia storage tanks is the main internal degradation mechanism which has to be taken into consideration when planning and executing an inspection programme. However, external factors for degradation, such as external corrosion, settling etc., have also to be considered. 16
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3.5.4 Low cycle fatigue Fatigue has been raised as a possible failure mechanism that may occur because of the long lifetime of an ammonia storage tank. Typical import tanks are filled and emptied every 1-2 weeks. The number of cycles during a lifetime is in the range 50 times/year times 40 years = 2000. Provided there are no significant defects present, this is far below the number of cycles that would be required to cause fatigue under normal operating conditions. Fatigue is therefore not considered to be relevant, unless special conditions may change the number of cycles or the stress levels are far above design specifications.
3.6 Indications from Accidents A survey of the AIChE Ammonia Safety Symposium proceedings was carried out in order to identify the important relevant factors which affect the integrity of tanks in practice. 18 incidents were found and their types of failures were identified and are listed in table 1. Table 1 Summary of incidents and basic causes Type of failure
Basic cause
Can it be discovered / prevented by internal or external inspection of the tank?
Number of occasions
SCC
Corrosion caused by combination of oxygen, ammonia, stress and carbon steel
Yes. However, intrusive inspection can be part of the cause
4
Filled annular space of double wall tank with ‘cup in tank’ design
Result is the floating of inner tank damaging the construction. a) Leak in the inner cup tank b) Ammonia condensation in annular space between the inner and outer tanks c) Splashing of ammonia over the edge of the inner tank
Yes, only when the root cause is a leaking inner tank. Otherwise, no, because the incident is caused by operational failures or design problems
3
Foundation failure due to frost heave
Freezing up and formation of ice lens under the tank caused by a) Defect of bottom heating tubes (two instances) b) Insulation defect caused by earthquake
By regularly checking the functioning of the bottom heaters this can be prevented. So this check must be part of the regular inspection programme
3
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Table 1 Summary of incidents and basic causes (continued) Type of failure
Basic Cause
Can it be discovered / prevented by internal or external inspection of the tank?
Number of occasions
Overpressure causing complete tank failure
a) Warm ammonia was injected in the tank. b) Sudden mixing of ammonia solution and liquid ammonia when an oil layer between these phases was broken up
No. Incidents were operational mistakes
2
Vacuum causing tank collapse
Failure of the pressure transmitters and a failing vacuum relief valve
The checking of the safety provisions must be part of the regular inspection programme
1
Failure of roof to wall weld
Poorly designed weld with high stress low cycle fatique
Yes
1
Leaking bottom
Improper welding techniques
Yes. In this case the leaks were detected by ammonia vapour around the tank
1
Leaking roof
Poor repair of a construction flaw
Yes
1
Leaking wall
Parent material was used that did not meet the specifications. Fatigue crack developed
Yes
1
Overflow from tank
Misunderstanding of level readings by operators in combination with failure of high level alarm
Operational error in combination with a defective high level alarm. The checking of the safety provisions must be part of the regular inspection programme
1
Historical records indicate that some major tank failures occurred due to a sudden pressure increase. This can happen for various reasons such as: • exothermic effect of mixing aqueous ammonia or water and anhydrous ammonia during tank commissioning (See Chapter 7, paragraph 3a) • unintentional injection of warm ammonia • flow of syngas due to gas break-through. 18
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The sudden pressure increase caused by the above mentioned failures are such that the pressure relief valves often cannot deal with the amount of gas. This can then result in a failure of the tank. When the weld between the tank bottom and the cylindrical shell is the weakest point, the shell will lift free from the bottom and the ammonia in the tank will be released. For this reason, it is preferable to include in the design of new tanks a weak point at the top to roof weld that fails before the bottom to cylindrical shell weld fails. Such a design change is not applicable to existing tanks. According to API 2000 emergency venting can be accomplished by a gauge hatch that permits the cover to lift under abnormal internal pressure. When a sudden pressure increase incident occurs that results in the lifting of the cylindrical shell, the liquid outlet piping that is attached to the cylindrical shell can also be damaged. Attention should be given to this possible failure scenario even when a double tank wall is used, since the liquid outlet piping also passes through the outer wall. A damaged outlet pipe can adversely affect the integrity of a double containment tank. The issue of weak roof to shell attachment is described in the section of API 2000 related to ‘Non Refrigerated tanks’ as well as API 650. For new tanks consideration should be given to the incorporation of this feature.
4. INSPECTION STRATEGY The inspection of low temperature ammonia tanks is a compromise between a need for knowledge about the tank condition and the negative effects of opening the tank for inspection, which will cause thermal stress and allow the ingress of oxygen. For ammonia tanks, it is known that decommissioning and recommissioning tends to increase the risk for SCC initiation. The need for inspection and its method, type and scope, therefore, should ideally be evaluated on the basis of the risk and consequence of a failure. Applying RBI means that these factors can be considered and the inspection programme can be established for each individual tank. In practice, however, frequencies of inspection may be ‘imposed’ by national legislation or industry code. A company may wish to follow a RBI approach and if the result of this is in conflict with the national legislation/code the company may consider taking up the matter with the relevant authorities. Various steps, which form the main elements of the RBI approach and methodology, are illustrated in Figure 8. It is essential that the design, construction and operating history of the tank are reviewed with the responsible engineers and operators during the formulation of an inspection strategy. It is also important to be familiar with and consider any local conditions that may influence the tank inspection programme: e.g. ambient conditions, local soil conditions, etc. 19
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RBI Process. Multidisciplinary Review
Identify Deterioration Mechanisms
Risk Analysis (Criticality)
Ranking incident scenarios
Risk Control Actions
Data Collection & Analysis
Design Production Inspection/Maintenance
Design or Process Improvements
Develop Inspection Plans
Data updating
Period / Type of Inspection Develop Validate Techniques
Repairs & Modifications
Execution of Inspection
Figure 8 Steps in RBI Process RBI and the associated structural integrity calculations can help to establish a tank inspection strategy that includes: • Definition of the most appropriate inspection methods. • Determination of the most appropriate tank monitoring requirements, including internal and external inspection aspects. • Establishment of prevention and mitigation steps to reduce the likelihood and consequences of a tank leak or failure. Different storage tank applications have unique design systems and conditions that must be considered when evaluating the tanks. It is therefore essential that experienced and competent engineers and inspectors are involved in evaluating existing tanks. The application of RBI to an ammonia tank requires an evaluation of the following factors: Failure Probability: 1. SCC related question 1.1. Oxygen and water content 1.2. Plate and weld material properties 1.3. Pipe connections
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1.4. Inspection issues 1.5. Repair 2. Other degradation mechanisms 2.1. External corrosion 2.2. Mechanical damage 2.3. Low cycle fatigue 2.4. Brittle fracture 2.5. Others 3. Operational issues 3.1. Pre-commissioning control 3.2. Commissioning procedure (inert purging, cooling rate) 3.3. Operating experience Failure Consequences: 1. Release of ammonia to the atmosphere, extra external safety (tank design, bund) 2. Leak before break assessment 3. Location of the tank (close to population and watercourse)
5. INSPECTION 5.1 Competence and Independence A high level of competence and experience is required in order to execute a thorough and effective assessment of the factors which may affect the integrity of tanks and the management of inspections. It is important that reliable data are used for the evaluation and it is essential that those involved have the required knowledge and experience to assess the influence of any uncertainties in the data used on the accuracy of the calculation. The application of fracture mechanics codes requires a high level of technical expertise and practical experience. Great care is essential in the selection of personnel to carry out such work. A group of people covering the areas of inspection, engineering/maintenance, operation and process safety should be involved in the evaluation. This team should have the appropriate degree of independence necessary to act impartially in all matters relating to the inspection of the tank.
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5.2 Assessment for Inspection Frequency The purpose of this assessment is to establish the basis for a risk based inspection programme. It covers the relevant parameters that can affect failure probability and failure consequences, as described in Section 3 and various safeguards described in Section 4. The evaluation will position each tank in an inspection frequency zone in an Inspection Frequency Diagram (Figure 9) as described below. The Inspection Frequency Diagram has been developed, based on results of surveys carried out within EFMA member companies. The 2007 survey covered 48 ammonia tanks (44 based in Europe, representing more than 80% of European capacity) and consideration of the prevailing legislation/standards. Whereas national codes are normally based on setting maximum limits for inspection periods, the diagram also provides a means of evaluating tanks that may be more vulnerable, where an improvement programme is required and where the period between inspections may need to be reduced. The diagram, therefore, provides a means of optimising tank inspections, intrusive or non-intrusive, based on RBI techniques and the practical experience of most European tanks. The Inspection Frequency Diagram is based on standard RBI processes that have been modified and developed to give a suggested inspection frequency for ammonia tanks based on an evaluation of failure probability and failure consequence factors. The detailed question list of the RBI assessment is given in Appendix 2. Each question is given a number of points between 0 and 10. The questions have been allocated different weighting factors to take account of the different levels of importance of the various factors considered. The score per question is a multiplication of the points and the weighting factor. The higher the score for each question, the higher is the probability or consequence of failure. The total score on the probability ranking is the sum of the scores of all the probability questions and this total will be between 0 and 100. The same holds for the total score for the consequence ranking. The probability of failure and the probability of consequences are plotted in the RBI matrix diagram in Figure 9. The matrix shows 5 levels of risk areas identified by different colours and these risk areas are linked to different inspection frequencies.
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Consequence of Failure
RISK MATRIX High risk Immediate mitigation actions shall be taken
81 - 100
RED
61 - 80
ORANGE
Medium High risk Limited inspection interval, maximum 5 - 10 years
41 - 60
YELLOW
Medium risk Average inspection interval, maximum 10 - 15 years
21 - 40
GREEN
Low risk Extended interval, maximum 15 - 20 years
DARK Very Low risk GREEN Extended interval, 20 - 25 years
0 - 20 0 - 24
25 - 36
37 - 48
49 -60
61 - 100
Probability of Failure
Figure 9 RBI Matrix: Inspection Frequencies as a function of failure probability and failure consequences 20-25 Years Very low risk. Indicates an ideal situation, where the tank properties are state of the art and the consequences of a failure are at a minimum. 15-20 Years Low risk. The tank has good properties and the consequence of a failure is well taken into account. Details about properties and/or location determine the given inspection interval. 10-15 Years Medium risk. The tank has acceptable properties. Details about properties and/or location determine the given inspection interval. 5-10 Years The tank has some elements in its design or the way it is operated that make an inspection necessary applying a higher frequency compared to the indicative industry average of 10-20 years. 19%. If after these steps some ammonia is still measured in the gas phase (for example, due to residual oil from which ammonia is slowly evaporating), breathing equipment must be used when entering the tank.
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8. GLOSSARY & EXPLANATION OF TERMS AE
Acoustic Emission
AIChE
American Institute of Chemical Engineers
API
American Petroleum Institute
AWS
American Welding Society
AWS 60xx
AWS (American) coding system for welding consumables
BBL
Break Before Leak
BS
British Standard
CIA
Chemical Industries Association (UK).
EFMA
European Fertilizer Manufacturers Association
EN E 46xx
EN (European) coding system for welding consumables
FCAW
Flux Cored Arc Welding
FSM
Field Signature Method
GTAW
Gas Tungsten Arc Welding
HAZOP
Hazard and Operability Study
Intrusive
Internal inspection by entering the tank
LBB
Leak Before Break
Liquefied
This term generally describes liquid ammonia which is close to its boiling point and is in contact with its vapour. Liquid, on the other hand, can describe sub-cooled as well as near boiling liquid.
MPI
Magnetic Particle Inspection
MT
Magnetic Testing
NDT
Non Destructive Technique
Non-intrusive Inspection of the internal condition of the tank from the outside RBI
Risk Based Inspection
SAW
Submerged Arc Welding
SMAW
Shielded Metal Arc Welding
SCC
Stress Corrosion Cracking
TOFD
Time of Flight Diffraction
UT
Ultrasonic Testing
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9. REFERENCES 1) Chemical Industry Association (UK). Guidance for the large scale storage of fully refrigerated anhydrous ammonia in the UK. June 1997. 2) Seveso Directive 2003/105/EC of the European Parliament and of the Council of 16 December 2003 amending Council Directive 96/82/EC on the control of major-accident hazards involving dangerous substances. OJ L 345 31. 12. 2003 pages 97-105. 3) API 620 R: Design and Construction of Large, Welded, Low-Pressure Storage Tanks. 4) EN 14620 series: Design and manufacture of site built, vertical, cylindrical, flatbottomed steel tanks for the storage of refrigerated, liquefied gases with operating temperatures between 0°C and -165°C. (Replaces BS 7777-1: Flat-bottomed, vertical, cylindrical storage tanks for low temperature service). 5) R. A. Selva and .M. Dickson. On the structural integrity of a refrigerated liquid ammonia storage tank at Norsk Hydro Fertilizers Ltd, NV. Report SS/870305’/RAS, April 1987. 6) R. Nyborg, P. E. Drønen and L. Lunde. Stress Corrosion Cracking in Low Temperature Ammonia Storage Tanks. AIChE Ammonia Safety Symposium, Vancouver, 1994. 7) R. Nyborg, L. Lunde and M. Conley. Integrity of Ammonia Storage Vessels – Life Prediction Based on SCC Experience. Material Performance, November 1991, p. 61 ff. 8) J. R. Byrne, F. E. Moir and R. D. Williams. Stress Corrosion in a Fully Refrigerated Ammonia Storage Tank. AIChE Safety in Ammonia Plants and Related Facilities Symposium, Denver 1988. 9) M. Appl, K. Fässler, D. Fromm, H. Gebhardt and H. Portl. New Cases of Stress Corrosion Cracking in Large Atmospheric Ammonia Storage Tanks. AIChE Safety in Ammonia Plants and Related Facilities Symposium, San Fransisco 1989. 10) R. A. Selva and A. H. Heuser. Structural Integrity of a 12,000-Tonne Refrigerated Ammonia Storage Tank in the Presence of Stress Corrosion Cracks. AIChE Safety in Ammonia Plants and Related Facilities Symposium, San Fransisco 1989. 11) S.B. Ali and R. E. Smallwood. Inspection of an Anhydrous Ammonia Atmospheric Pressure Storage Tank. AIChE Safety in Ammonia Plants and Related Facilities Symposium, San Diego 1990. 12) R. Nyborg and Liv Lunde. Measures for Reducing Stress Corrosion Cracking in Anhydrous Ammonia Storage Tanks. AIChE Ammonia Safety Symposium, Vancouver, 1994. 39
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13) M. Walter and R. Lesicki. Measures taken to ensure safe operation of ammonia storage tank. Ammonia Technical Manual 1998, p. 127 ff. 14) L. Lunde and R. Nyborg. Stress Corrosion Cracking of Carbon Steel Storage Tanks for Anhydrous Ammonia. Proceedings of the International Fertiliser Society, York, UK. No. 307, 1991. 15) S. Hewerdine. Ammonia Storage Inspection Proceedings of the International Fertiliser Society, York, UK No. 308, 1991. 16) L. Lunde, R. Nyborg and P. E. Drønen. Control of Stress Corrosion Cracking in Liquid Ammonia Storage Tanks. Proceedings of the International Fertiliser Society, York, UK No. 382, 1996. 17) EFMA Meeting (Internal), European ammonia storage meeting Proceedings, Oslo, November 1999. 18) AIChE 50 years proceedings CD-ROM, 2005. 19) BS 7910: Guide to methods for assessing the acceptability of flaws in metallic structures. July 2005, ISBN: 0580459659. 20) EN 13445-2:2002/A2:2006: Unfired pressure vessels – Part 2: Materials (German version). 21) API 579: Recommended Practice for Fitness-For-Service and Continued Operation of Equipment. 22) EN 473: Non-destructive testing. Qualification and certification of NDT personnel. General principles, December 2000, ISBN 0 580 36795 9.
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APPENDICES APPENDIX 1
Welding consumables for ammonia tank construction
42
APPENDIX 2
Risk based inspection evaluation
43
APPENDIX 3
Crack configurations that should be evaluated by structural integrity calculations
46
Analysis of oxygen in liquid ammonia
47
APPENDIX 4
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APPENDIX 1: WELDING CONSUMABLES FOR AMMONIA TANK CONSTRUCTION STRENGTH LEVEL
“LOW” STRENGTH
“MEDIUM” STRENGTH
42
TYPE OF WELDING CONSUMABLES
STANDARD/ GRADE/ DESIGNATION
TYPICAL YIELD STRENGTH [MPa]
TYPICAL TENSILE STRENGTH [MPa]
SMAW
AWS E60xx
min. 331
min. 414
FCAW
AWS E6xT-x
min. 345
min. 428
SAW
AWS F6x-Exxx
min. 330
415-550
SMAW, FCAW and SAW
EN E 38 x x x x
min. 380
470-600
SMAW
AWS E70xx
min. 390
min. 480
FCAW
AWS E7xT-x
min. 414
min. 497
SAW
AWS F7x-Exxx
min. 400
480-650
SMAW, FCAW and SAW
EN E 42 x x x x
min. 420
500-640
SMAW
AWS E80xx
min. 460
min. 550
SMAW, FCAW and SAW
EN E 46 x x x x
min. 460
530-680
1-5
1-4
1-3
SCC Oxygen / Water content
LS (SMYS 300 and < 360Mpa) HS (SMYS >360Mpa)
Repairs after first commissioning
Have you experienced SCC
No Yes without welding Yes with welding minor repairs with stress relief minor repairs without stress relief major* repairs with stress relief major* repairs without stress relief *major means extensive areas or deep grinding (>20% of the wall thickness)
0
0
0
0
0
Points
Total Score SCC
2 6 4 10
0 2
5 10
Yes, but small number of defects, < 5 defects and scope in accordance with EFMA recommendations. Yes, several defects
Repairs
0 5 10
0/5 0/5
0/2 0/2
0/2/4
0 1 2
0 5
0 5
10
0
Possible Scores
No, inspection scope in accordance with EFMA recommendations No, inspection scope not in accordance with EFMA recommendations No, but no inspection for SCC done
Inspection issues
Pipe connections pre-made and stress relieved before installation (Yes / No) Manhole pre made + stress relieved before installation (Yes / No)
Manhole and pipe connections
LS / MS / HS Have you performed and documented internal welding control before commissioning the tank for the first time ?(Yes / No) Did you carry out repairs during pre-commissioning ?(No/Yes)
Welding consumables (see appendix 1)
Plates
Materials, welding and pre-commissioning.
Follow up of Oxygen / Water content (particularly in case of imported NH3) Only own ammonia frequent (1x month) not frequent Imported ammonia frequent (every shipload or every 50 rail tank cars) not frequent
Do you add water >0.2 % If yes, score is 0 If no, continue with the following question Is water / oxygen content in the safe area (see figure 5)., If no, or not analyzed, score is 10, If yes, continue with the following question
Weighting Factor 0.5
1.5
0.5
1.5
2
0
0
0
0
0
0
60
0%
0%
0%
0%
0%
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Probability of Failure
Total
(This appendix is electronically available as an Excel file on EFMA’s website www.efma.org)
APPENDIX 2: RISK BASED INSPECTION EVALUATION
% of total
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43
2-1
2-2
2-3
2-4
2-5
2-6
2 No Yes, corroded location was recoated Yes, no coating applied No Yes, with well-controlled repairs Yes, without repairs
External corrosion
Other Degradation Mechanisms
Mechanical damage
Low Cycle Fatique Brittle fracture
Flexible pipe connections (Yes / No) Variations in ammonia level to minimum and maximum, frequent (>15 x a year) / non frequent
External conditions
Do you have material certification, or have you performed a Charpy test at -33°C (or lower) using material with the same charge number that satisfies the requirements in the design code? (Yes / No)
Previous Inspections
External factors like extreme weather conditions leading to stress If no, score is 0 If yes, but additional constructive improvements or simulations made score is 2 If yes, score is 4 Have you observed settling/subsidence phenomena? No, score is 0 Yes, but additional constructive improvements or simulations made score is 2 Yes, score is 6 Have you performed any internal inspections after commissioning? Yes/No
0 / 2 /6
0/2/4
0 / 10
0/5 5/0
0 5 10
0 4 10
Possible Scores
0
0.2
0
0%
0%
0.2
0
0
0%
0.2
0
0
0%
0.8
0
0
0%
0.2
0
0
0%
% of total
0
Total
20
Weighting Factor
0
Points
0 0.4 0 / 10 Total score Other degr.
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Commissioning procedure
Have you experienced operating problems that may have resulted in unfavourable conditions in the tank (e.g. lifting of relief valves, over-/underpressure, overfilling,prolonged failure of floor heating etc.). If one of these incidents has occurred : score is 10
Operating experience
Do you have a procedure for (de)commissioning (yes/no) Is the procedure in accordance with this guideline: Decommissioning Evaporate the remaining ammonia in a way that ensures uniform and slow heating, not exceeding 1 °C/hour.(yes/no) Purge with warm ammonia gas or nitrogen until all liquid ammonia is removed. The bottom area may need to be cleaned before it is possible to get all the ammonia gas out.(yes/no) Recommissioning Purge with nitrogen until the measured oxygen in the discharge gas is less than 4%.(yes/no) Purge with ammonia gas until the measured oxygen in the discharge gas is less than 0.5%.(yes/no) Cool the tank down to as low a temperature as possible, at a cooling rate lower than 1 °C/hour. Use preferably a spray system.(yes/no)
Hydro tested (Yes / No)
Operational issues Pre commissioning control
Possible Scores
Points
0
0
0
0.4
1.2
0.4
Weighting Factor
0 / 10 Total score operational issues
0/2
0/2
0/2
0/2
0/2
0 / 10
0 / 10
Total
0
0
0
0
% of total
20
0%
0%
0%
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3-1
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2
1
2-1
3
10
2
2
6
8 10
0 7
10
10 10
0 2
5
8 7
0
0
0%
0%
5
0
0%
0
2.5
2.5
0
% of total
0
Total
100
Weighting Factor
0
Points
Consequence of Failure
Consequence Ranking
Leak before break assessment
e) Single steel tank without bundwall (see fig. 6)
d) Steel tank with an embankment of earth to contain full contents of the tank and no roof over the space between the tank and the embankment.(fig.5)
c) Steel tank with a partial height concrete bund wall with impervious floor within the contained area and no roof over the space (see fig.4)
b) Steel tank housed within another steel tank to contain full contents of the tank, with a single roof (cup in tank) or independent roofs (see Figures 3 );
a) Steel tank with full height concrete bund wall close to it with capacity to contain full contents of the tank and the space between the tank and the bund having impervious floor and roof covering (see Figure 2)
Release of Ammonia to the atmosphere / Extra external safety
Assessment performed Yes, conclusion "Leak before break"
Yes, conclusion "Break before leak"
Yes, but "Non-conclusive"
No
Location of the tank Not close to external population (2km), not close to water (200m.) Not close to population and close to water Close to population between 1 and 2 km, not close to water Close to population less than 1 km, not close to water
Close to population between 1 and 2 km, close to water Close to population less than 1 km, close to water
Possible Scores
10 Total score Consequence of Fai
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APPENDIX 3: CRACK CONFIGURATIONS THAT SHOULD BE EVALUATED BY STRUCTURAL INTEGRITY CALCULATIONS
The sketch above illustrates the typical defect orientation and locations which should be considered when maximum tolerable defect size calculations are carried out. When necessary, the calculated results for these locations can be interpolated for shell courses 2 to n-1. A1 (n): Transverse cracks in vertical welds in course 1 (course n). B1 (n): Longitudinal cracks in vertical welds in course 1 (course n). C1 (n): Transverse cracks in horizontal welds between courses 1 and 2 (courses n and n+1), located at a T-weld. D1 (n): Transverse cracks in horizontal welds between courses 1 and 2 (courses n and n+1). E1 (n): Longitudinal cracks in horizontal welds between courses 1 and 2 (courses n and n+1). Course n is at a level higher in the tank wall, e.g. with lower strength material. 47
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APPENDIX 4: ANALYSIS OF OXYGEN IN LIQUID AMMONIA 1. Introduction In the assessment of the risk for stress corrosion cracking, the oxygen concentration in the liquid ammonia is a key parameter and should therefore be closely monitored. This appendix describes some guidelines for analysing the oxygen content in the range relevant for stress corrosion cracking (0.1-10 ppm).
2. Sampling The key parameter for stress corrosion cracking is the oxygen content in the liquid ammonia. It is important to realise that the oxygen distribution between the liquid and the vapour phase is such that, in equilibrium at -33°C, the oxygen concentration in the vapour is about 10.000 times higher than the oxygen concentration in the liquid. At temperatures > 0°C this factor decreases to 100-1000. In cases where equilibrium can be assumed between the liquid and the vapour phases, it is recommended to analyse the vapour phase as the method applied can be less sensitive due to the much higher concentration in the gas phase. Furthermore it is important to note that when a liquid sample is taken and vaporised, the total sample shall be vaporised and analysed in order to obtain representative results.
3. Analysis methods When analysing oxygen in the relevant range, it is of vital importance to prevent sample contamination by traces of oxygen in the carrier or purge gases used during the various steps. Several methods are applied within the industry to analyse oxygen in ammonia in the required concentration range: Method
Detection limit (ppmv)
Gas chromatograph followed by mass spectrometer
0,1
Gas chromatograph followed by Thermal conductivity detector
1
Gas chromatograph followed by Electron Capture detector
2
Analyser based upon fuel cell, creating an electric current, proportional to the oxygen concentration
1 (Manufacturer claims lower values)
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